1. Field of the Invention
This invention relates to a cable joint, for example a low voltage, 1 kV joint, and in particular to a cable joint, for example a branch joint, that is substantially sealed against the ingress of moisture, for example a joint having sealant material retained within a heat-recoverable sleeve or other housing that applies pressure to the sealing material during installation of the joint.
2. Description of the Prior Art
Joints in cables, whether power or communications cables, and whether in-line or branch joints, often need to be sealed against the ingress of moisture, either as a vapour or a liquid, which may be under pressure. Furthermore, for multicore power cables operating at voltages of 1 kV or higher, it is necessary to fill the joint so as to prevent any moisture that may have penetrated the cable from reaching the electrically-vulnerable joint region. Such moisture could have a deleterious effect on the joint and eventually lead to its electrical failure by breakdown from operating voltage to earth potential. These sealing problems are particularly acute in the case of a power cable, which undergoes thermal cycling and in continuous operation can reach a conductor temperature of 90.degree. C., or even 130.degree. C. under certain conditions. The pressure pumping effect of the thermal cycling, combined with the high temperature can allow any water that is in the cable to enter the joint and potentially fore a short circuit, and can also produce voids arising from breakage of interfacial sealing with the cable cores, and also from the sealant material being urged away from the conductors and even from the joint region and along the cable. Such problems are made even worse in the case of a branch joint, since the volume to be filled is comparatively large and has a complicated geometry, thus making it more difficult to ensure sufficient sealing.
Sealed joints can be of various types, for example: (1) a rigid box that is filled with a liquid composition that solidifies, such a composition may be poured in hot, for example bitumen, or may comprise a cold curing system, for example polyurethane resin; (2) a rigid box that is filled with a liquid composition that forms a flexible skin upon contact with air or moisture; (3) a taped housing into which is injected a liquid composition that solidifies: (4) a heat recoverable sleeve having internally coated thereon a mastic or hot melt sealant which becomes liquid upon heating the sleeve and flows to fill the joint; and (5) a two part housing that is filled with grease or a putty-like filler that is squeezed into voids within the joint region upon closing together of the two parts by mechanical means, the housing may be subsequently contained within a heat recoverable sleeve that seals on to the cable jackets. The types (2) and (5) have no, or only very limited, resistance to water pressure, and these and other types do not always reliably fill the joint region, especially at low ambient temperatures.
Various joint constructions have been proposed to achieve water blocking and void filling. WO 91/00601 (Raychem) discloses an electrical cable in-line or branch joint in which a first, hydrophobic sealant material is located around the exposed electrical connections of the cable conductors to act as a water seal, and in which a second, blocking material, such as fibrous material, is located in the crutch region of the joint to prevent the hydrophobic material being physically displaced away from the electrical connections by pressure of water within the cable, or by pressure within the material generated by load-cycling of the cable. The joint region is encapsulated within a heat-shrinkable polymeric sleeve that is selected to be resistant to higher temperatures so that sufficient heat can be applied to it to cause the sleeve to recover into sealing contact with the cable jackets, and to exert sufficient pressure on the sealant material and to transmit sufficient heat thereto such that the sealant melts and flows into all the interstices in, between, and around the conductors and the connector so as to fill any voids.
EP-A-0 153 174 (Raychem) discloses a medium voltage power cable joint in which an insert is disposed between connected conductors of two cables; At least the outer surface of the insert comprises a void filling material, and so overcomes the problem of getting the sealant into the relatively inaccessible region between the cores. Further portions of void-filling material are placed around the outside of the conductors, and the joint region is then enclosed within a heat recoverable polymeric sleeve. The preferred void-filling material is a thermoplastic material having a viscosity at 70.degree. C. of between about 1.times.10.sup.3 and about 1.times.10.sup.5 poise. On application of heat to effect recovery of the sleeve, the void filling materials melt and the pressure of recovery of the sleeve forces the material to fill all the spaces between the insert, the conductors and the sleeve.
GB-A-2 100 281 (Raychem) discloses a hydrophobic filling composition based on mineral oil for sealing connections between telecommunication cables. The filler is contained within a polymeric wrapper and is manually urged into the interstices between the plurality of conductors. The splice region is finally enclosed within a heat-recoverable polymeric sleeve.
GB-A-1 177 915 (Lacal Industries) discloses a cable joint between two single-core cables in which a non-hygroscopic, non-melting, water-repellent grease is urged by the recovery of a heat-recoverable sleeve around and into the interstices of an exposed stranded conductor.
GB-A-2 099 639 (Tatsuta Electric Wire and Cable) discloses a gas-tight cable joint wherein a resin filler is retained within a heat shrinkable sleeve around the joint in which a sealing tape is wound around the cut back ends of the cables to enhance the pressure sealing.
EP-B-0 115 220 (Minnesota Mining and Manufacturing) discloses a pressure tight termination of a pressurised long distance cable in which a sealing composition, which may be a thixotropic polyurethane-based material, is applied to bared wires of the cable, and is cured by the heat from an externally-applied heat-shrinkable sleeve.
WO 87/07779 (Raychem) discloses a waterblocked telecommunications cable in which void-filling material, such as foam ethylene propylene diene rubber or gel, is inserted between and around groups of a plurality of insulated conductors. A tape is then wrapped around the void-filling material so as to prevent its migration along the greasy conductors, and the tape itself may then be contained within a cast material or a heat-recoverable sleeve.